The present invention relates to a method and an apparatus for preventing biofouling in a cooling water system. More particularly, it relates to a biofouling preventing method for a cooling water system for preventing biofoulings in a closed cooling water system having a cooling tower, and to an apparatus therefor.
Water is used in large quantities in many kinds of industry and other fields for many purposes, wherein it is mostly used as cooling water. While there are known transient cooling methods in which cooling water heated through heat exchange or the like is expended, a method in which water is circularly used by employing a cooling tower and decreasing the water temperature utilizing temperature differences between air and water or vaporization is used in cases where available water resource is limited. It is in many cases possible to lower the water temperature by approximately 1.degree. C. with a water consumption of approximately 2% in such a circulation method, which might differ depending on various conditions. In view of effective utilization of water resources, such a cooling method by means of a closed cooling water system is quite important.
Such a biofouling preventing apparatus for a cooling water system employing a typical closed cooling water system with a cooling tower is known to comprise, for instance, the following components as shown in FIG. 5: a cooling tower 53 including a ventilating fan (fan) 50, fillers 51, and a reservoir 52; a heat exchanger 54; a circulating pump for cooling water 55; an injector for agents 56; a reservoir for agents 57; a fresh water supplying line for supply 58; and a drawing line for circulating water 59.
In such a biofouling preventing apparatus for a cooling water system, the cooling water circulates through the heat exchanger 54 and the cooling tower 53 by means of the circulating pump 55. The water which has been heated by the heat exchanger 54 is lowered in temperature by being forced to come in contact with an air flow from the ventilation fan 50 while it is dispersed to the filling material in the fillers 51 through a dispersing device at a top portion of the cooling tower whereupon it forms a film of water at the wall surface of the filling material and drops. The water then passes through the reservoir 52 and is again sent to the heat exchanger 54, and this process is repeatedly performed thereafter. Since the water is essentially forced to evaporate and is dispersed in the cooling tower 53, fresh water corresponding to at least this loss amount is supplied from the line 58. Further, in order to prevent accumulation of substances such as minerals in the circulating water which do not evaporate, a part of the circulating water is drawn from the line 59 as drawing water. Therefore, it is necessary to supply fresh water to compensate for this amount.
As circulating water comes into contact with a large quantity of air in this manner, various microorganisms included in the air are mingled into the system which reside, multiply and are stored therein. Such microorganisms adhere to filling materials of the cooling tower, the heat exchanger surface, or other channel walls to form a biofilm and might cause plugging, decreases in heat exchange efficiency and corrosion of composing materials. In addition, there exists the danger that virus, a representative example of which is Legionella Pneumophila, might be discharged into the air through the ventilation fan of the cooling tower.
Such hazards are generally called a biofouling. In order to prevent such a biofouling, bactericides are injected into the circulating water. Mainly used bactericides are chlorinous agents. In this case a bactericide which is input into the injector 56 through the reservoir 57 generally dissolves into fresh water and is continuously or intermittently (in an almost continuous manner) injected to the cooling tower 53, as shown in FIG. 5.
When using such bactericides such as chlorinous agents, the density of the bactericide in the cooling water needs to be maintained at a high level to obtain sufficient effects. This, in turn, presents drawbacks such as corrosion of composing materials of the cooling water system, influences to peripheral environments of droplets in the cooling tower, and environmental pollution due to residual bactericide in the drawing drainage or byproducts of the bactericide such as chlorinated hydrocarbon. Further, since chloride is accumulated in the circulating water, the amount of drawing water needs to be increased to prevent the accumulation.
On the other hand, in some countries such as in the U.S.A., there are cases in which ozone is employed instead of chlorine. As compared to chlorinous agents, ozone is decomposed or consumed in water in a relatively short time. For instance, an ozone injection method might be employed instead of the above agent injecting method, as shown in FIG. 6. In FIG. 6, numeral 60 denotes an ozone generator, 61 an ozone injector such as ejector, and 62 a pump for drawing circulating water. The cooling water in the reservoir 52 of the cooling tower 53 is drawn by the pump 62, a vaporized ozone-containing gas generated by the ozone generator 60 is dissolved in the cooling water drawn by the injector 61 and returned to the reservoir 52. The reason why a different injection method is taken than for chlorinous agents is that ozone is unstable and highly reactive, whereby it is smoothly decomposed and vanishes in water. Generally, it is continuously injected. While the amount of injection of ozone depends on quality of cooling water, an amount of 1.7 to 80 g per day with respect to 1 t of held cooling water is required (Federal Technical Alerts, U.S. Department of Energy, A. E. Pryor and M. Fisher, Ozone Science & Engineering 16(6), 505-536 (1994)).
When employing ozone, a suitable amount of injected ozone solves many problems which arise when employing bactericides such as chlorinous agents. However, drawbacks are presented in that costs for the ozone generator and for efficiently generating ozone are relatively high and in that the dissolved ozone density in the cooling water is limited since ozone in the water is easily diffused into air so that ozone can not be injected by an amount as required for presenting sufficient effects. These drawbacks result in higher costs for facilities and maintenance.
It might be considered to connect, for instance, a microorganism removing apparatus which is employed in cooling water piping for power stations (refer to Japanese Examined Patent Publication No. 2559/1984) instead of the above-described apparatus of the ozone injecting method. However, by employing this removing apparatus for injecting ozone into cooling water which is drawn by the pump 62, it will be made to flow out of the line 59, and thus will not be economical and not suitable for efficiently preventing biofoulings.
The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a biofouling preventing method for a cooling water system and an apparatus therefor with which biofoulings can be reliably prevented.